You never got any monasteries in the whole game, so I'll have to assume you do mean a cathedral.
When reporting a bug, it is wise to give more information so the developers can track down what you are referring to. Like a turn number would be useful. I went through the whole game but sure wouldn't want to do that for every bug reported. (i.e. try to keep the person in mind who has to try to find this.)
I'm assuming you're referring to move 178 when you placed the cathedral. That's simple: you were already on the city and cannot place another meeple in a direct connection like that. I don't know this expansion but I looked up the rules from Z-Man and it specifically says all placement rules remain the same as for the base game. That's why you can't place a meeple on there. No bug. And I noticed both expansions were using tiles only so additional rules wouldn't count anyway.
(I don't know about any rule that says you cannot place on a cathedral at all, because that isn't in the rules I read, so not sure what DrKarotte refers to, but I don't think it's relevant since it would have been an illegal placement anyway.)
If you're referring to something else, I can't find it and you'll need to be far more specific.
I guess this is not a bug afterall. Thanks for the clarification. 
